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Summary of Significant Accounting Policies - Summary of Fair Value Measurement Financial Asset and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets [Abstract]
Cash and cash equivalents	$ 62,500	$ 50,800
Total assets measured at fair value	155,910	251,666
Money market funds
Assets [Abstract]
Cash and cash equivalents	61,512	46,781
U.S Treasury securities
Assets [Abstract]
Marketable securities	3,990	8,826
Government-sponsored enterprises
Assets [Abstract]
Marketable securities	11,990	29,759
Corporate debt securities
Assets [Abstract]
Marketable securities	78,418	166,300
Quoted Prices in Active Markets for Identical Assets, Level 1
Assets [Abstract]
Total assets measured at fair value	61,512	46,781
Quoted Prices in Active Markets for Identical Assets, Level 1 | Money market funds
Assets [Abstract]
Cash and cash equivalents	61,512	46,781
Quoted Prices in Active Markets for Identical Assets, Level 1 | U.S Treasury securities
Assets [Abstract]
Marketable securities	0	0
Quoted Prices in Active Markets for Identical Assets, Level 1 | Government-sponsored enterprises
Assets [Abstract]
Marketable securities	0	0
Quoted Prices in Active Markets for Identical Assets, Level 1 | Corporate debt securities
Assets [Abstract]
Marketable securities	0	0
Significant Other Observable Inputs, Level 2
Assets [Abstract]
Total assets measured at fair value	94,398	204,885
Significant Other Observable Inputs, Level 2 | Money market funds
Assets [Abstract]
Cash and cash equivalents	0	0
Significant Other Observable Inputs, Level 2 | U.S Treasury securities
Assets [Abstract]
Marketable securities	3,990	8,826
Significant Other Observable Inputs, Level 2 | Government-sponsored enterprises
Assets [Abstract]
Marketable securities	11,990	29,759
Significant Other Observable Inputs, Level 2 | Corporate debt securities
Assets [Abstract]
Marketable securities	78,418	166,300
Significant Unobservable Inputs, Level 3
Assets [Abstract]
Total assets measured at fair value	0	0
Significant Unobservable Inputs, Level 3 | Money market funds
Assets [Abstract]
Cash and cash equivalents	0	0
Significant Unobservable Inputs, Level 3 | U.S Treasury securities
Assets [Abstract]
Marketable securities	0	0
Significant Unobservable Inputs, Level 3 | Government-sponsored enterprises
Assets [Abstract]
Marketable securities	0	0
Significant Unobservable Inputs, Level 3 | Corporate debt securities
Assets [Abstract]
Marketable securities	$ 0	$ 0

X

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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